Paper or paperboard structures often include a plurality of sheets of material adhered in face-to-face contact. For example, most envelopes, cartons, display structures, and the like include seams or reinforced areas where at least portions of adjacent sheets of paper or paperboard material are adhered together.
The required strength of the adhesive bond between adjacent sheets of paper or paperboard is dependent upon the intended end use of the completed structure. This bond strength is affected by many factors including: the respective characteristics of the paper or paperboard being adhered; the type of adhesive; the rate of application of adhesive; the temperature of the adhesive at the instant of application; the pattern by which the adhesive is applied; the elapsed time between the application of the adhesive to the first sheet and the contact of the second sheet thereto; and the amount and duration of pressure applied in securing the two sheets together.
The many factors that affect adhesive bonds make it difficult to precisely determine the specific parameters for adhering sheets of paperboard to one another. The determination of specific adhesion characteristics is particularly difficult in view of the infinite number of possible bond strength requirements, and the everchanging availability of adhesives on the market. Consequently it is necessary for the manufacturer of paperboard structures to undertake a significant amount of testing to determine the proper bonding parameters.
Automatic testing equipment is available for experimenting with and determining the proper adhesive application and bonding parameters. For example, one such piece of testing equipment is shown in U.S. Pat. No. 3,490,278 which issued to Van Saun et al on Jan. 20, 1970. The apparatus shown in U.S. Pat. No. 3,490,278 is directed to a machine including a chain and a pusher for advancing a sheet of paper or paperboard material in a longitudinal direction along a test path. A hot melt adhesive applicator is disposed in the test path of the prior art machine. The applicator is connected by conventional means to a hot melt adhesive auxiliary unit. This auxiliary unit on the prior art test machine is spaced from the applicator and includes controls that can be used to adjust the temperature of the adhesive in the auxiliary unit and to vary the rate of flow of adhesive from the auxiliary unit. Thus, the testing machine shown in U.S. Pat. No. 3,490,278 provides a crude apparatus for controlling the temperature and flow rate of adhesive to an adhesive applicator spaced therefrom. The machine of U.S. Pat. No. 3,490,278 further includes a pressure mechanism for holding a second sheet of material by vacuum. An appropriate external signal will cause the pressure elements to advance toward one another, thereby placing the two sheets into face-to-face contact with the adhesive disposed therebetween. The amount of pressure and the duration of pressure application can be externally varied.
The machine of U.S. Pat. No. 3,490,278 has several significant deficiencies. Specifically, it is now known that the precise temperature at which a hot melt adhesive is applied can significantly affect the bonding characteristics. However, the machine of U.S. Pat. No. 3,490,278 only attempts to control the hot melt adhesive temperature in an auxiliary unit that is spaced from the actual applicator of adhesive. In travelling to the actual applicator, the hot melt adhesive temperature can vary significantly, thereby affecting the quality of the adhesive bond. Additionally, glue disposed adjacent the nozzle can cool substantially between adjacent tests. Since the test bead is usually quite short, a large part of each test bead can be much cooler than the heated glue in the auxiliary unit. Another deficiency of the machine shown in U.S. Pat. No. 3,490,278 is the inclusion of a large number of manually controllable externally initiated operations. Consequently there is a substantial probability of significant variations from one test to the next, thereby giving inaccurate or misleading test results.
Another testing machine is shown in U.S. Pat. No. 3,444,732 which issued Robbins et al on May 20, 1969. The testing apparatus shown in U.S. Pat. No. 3,444,732 addresses the importance of temperature control, but only is concerned with that part of a manufacturing process where heat and pressure are applied to a preexisting bead of glue. U.S. Pat. No. 3,444,732 does not address the step-by-step analysis and testing of other factors involved in the manufacture of paper or paperboard structures, and further is not concerned with the accurate testing and control of conditions related to the application of adhesives.
In view of the above, it is an object of the subject invention to provide an apparatus for testing the various parameters that are essential to accurate adhesive bonding during the manufacture of paper or paperboard products.
It is another object of the subject invention to provide an apparatus for automatically and reliably testing the rate and temperature of adhesive as it is being applied to a sheet of paper or paperboard material.
It is an additional object of the subject invention to provide an apparatus for accurately testing and controlling adhesive application and subsequently testing and controlling the amount and duration of pressure between two sheets of material being adhered to one another.
It is a further object of the subject invention to provide an apparatus capable of automatically and reliably repeating or varying a plurality of testing parameters.
It is still another object of the subject invention to provide an apparatus that will ensure that glue temperature will remain constant throughout each test and can be controlled from one test to the next.